Before explaining the prior art, description will be given on general features of automatic gain control in a hearing aid. In a hearing aid, it is necessary to change the gain according to the level of the input sound pressure (and not to evenly amplify the input sound). In case of a user, who is accompanied with recruitment phenomenon, it is difficult to hear low sound, while big sound or strong sound can be heard on the same level as a person with normal hearing ability. Thus, it is necessary to increase the degree of amplification when the input sound is low, and to suppress output level without amplifying very much when the input sound is high or strong and to suppress the output level to the range where the user can hear. For instance, when impact sound of shutting a door is inputted and if it is amplified in the same manner as low input sound, the output level gives uncomfortable feeling to the user, and the output level must be suppressed. In case of big or strong sound of conversation is heard, signal level reaches saturation range of the amplifier or the earphone, and the output sound is extremely distorted, and this gives influence on sound quality and hearing of speech sound.
For the purpose of suppressing the impact sound or big or strong sound (excessive sound) at input stage or output stage of the amplifier depending on the signal level, an automatic gain control circuit or a peak clipping circuit is used. In case the peak clipping circuit is used, a limiter is applied on waveform of more than a predetermined level. As a result, harmonic distortion occurs, and this decreases speech sound articulation of consonant. Thus, the automatic gain control circuit is effective. In the automatic gain control, input signal or output signal is monitored, and a signal exceeding a predetermined level is compressed. In this function, setting values for the attack time from the start of the operation and the release time up to the release of the operation are important. When the attack time is long, impulse-like signal such as impact sound cannot be suppressed, and uncomfortable sound is outputted. In case the attack time and the release time are longer, speed sound is heard as if it is rebounded. Thus, in the automatic gain control of a hearing aid, it is effective to control and to shorten the attack time and the release time.
The automatic gain control known in the past is described in the patent reference 1 as described below. Description will be given below on the automatic gain control known in the past. FIG. 3 is a circuit diagram showing an arrangement of an automatic gain control circuit of a hearing aid. In FIG. 3, a sound signal inputted via a microphone 10 is amplified with a certain fixed gain by an amplifier 30 and is outputted to an earphone 100. A variable resistance 140 takes out a part of AC voltage between both ends of the earphone 100. A diode 150 and a capacitor 155 rectify the partial AC voltage between both ends of the earphone 100. Output terminal of RC filter, which comprises a resistance 161 and a capacitor 162, is connected to a base of a first transistor 170. A collector of the first transistor 170 drives the base of a second transistor 180. The second transistor 180 is used to short-circuit the input signal of the amplifier 30 to the grounded side. A battery 80 is used as a power source of this hearing aid. Resistances 230, 240, 250, 260, 270 and 280 are used to determine the level to start compression and to determine input/output characteristics.    Patent Reference 1: Patent Application Publication JP-A-58-162115 (FIG. 1)
Now, description will be given on operation of the automatic gain control circuit with the above arrangement. When a signal between both ends of the variable resistance 140 is increased, the base of the second transistor 180 connected to input terminal of the amplifier 30 is directly driven by the collector of the first transistor 170, and the input signal of the amplifier 30 is short-circuited over a wide range by the second transistor 180. Base voltage of the second transistor 180 is changed depending on signal level at the output stage of the amplifier 30. The signal at the input stage of the amplifier 30 is attenuated, and the function of the automatic gain control is fulfilled.
However, in the automatic gain control circuit known in the past as described above, for the purpose of attaining sufficient effect by RC filter, which comprises a resistance 161 and a capacitor 162, and also for the purpose of following up an impulse-like signal such as impact sound, it is necessary to have longer time constant for RC, while it is difficult to accommodate large RC component in a small housing of a hearing aid. As a result, the impulse-like signal such as impact sound cannot be followed up, and the function of the automatic gain control may not be sufficiently fulfilled.